1. Field of the Invention
The present invention relates to a thread forming apparatus having tangential chasers for cutting a thread on a railroad spike. More particularly, the present invention relates to a thread forming apparatus having tangential chasers with particular cutting edge profiles for tapering and cutting a thread on a large railroad spike wherein the thread has a pitch crest diameter, root and a relatively large gap between successive turns of the thread, all of which remain constant for the length of the thread.
2. Description of Related Art
A railroad spike fastens a rail to a wooden tie of a railroad. A commonly used railroad spike is known as the "Tirefond" or "North American" railroad spike which is illustrated in FIG. 1. That spike includes a tool receiving head 10 and a shank 12 with a tip 13. The shank has a cylindrical portion 14 and a tapered portion 16 which increases in diameter from the cylindrical portion 14 upwardly toward the head 10. The diameter of the cylindrical portion is preferably about 0.657 to 0.664 inches while the diameter of the tapered portion adjacent the head is about 0.924 inches. Preferably, the length F1 of the tapered portion 16 of the shank is about 1.75-2.00 inches, while the length F2 of the cylindrical portion 14 is about 2.5 inches.
The shank 12 is threaded across the cylindrical and tapered portions so that a tool acting on the head can screw the spike into the tie. When the spike is screwed fully into the tie, a collar 18 located between the head 10 and the tapered portion 16 of the shank caps the screw hole to prevent ingress of water and other corrosives.
The spike must resist forces tending to pull-out or loosen the spike due to vibrations caused by trains passing over the rails. Accordingly, the thread arrangement on the shank is critical to the resistance of the shank to pull-out and loosening. The thread form of the Tirefond or North American spike (illustrated in FIG. 1) has its thread beginning at the top of the tapering portion 16 and helically progressing downward toward the tip 13 of the cylindrical portion 14 of the shank. The angle which the flank of the thread facing the head (i.e., the upper flank) makes with the normal to the axis of the spike is smaller than the angle which the flank of the thread facing the tip of the spike (i.e., the lower flank) makes with the normal. The length of the root R or base of the thread is constant and is preferably about 0.157 inches. The height K of the threads from the surface of the cylindrical portion of the shank to the crest of the thread is preferably about 0.128 inches. The spike has a constant crest diameter so that the height of the threads from the surface of the spike decreases as the thread progresses up the tapering portion of the spike. The crest diameter H is preferably between 0.913 and 0.920 inches.
In the railroad spike illustrated in FIG. 1, the pitch P (i.e., the distance between corresponding portions of adjacent turns of the thread) is constant and relatively large, for example 0.5 inches and preferably about 0.492 inches between adjacent thread crests. A gap G exists between the upper flank of one thread turn and the lower flank of an adjacent thread turn. The gap G is constant for all thread turns and has a length of about 0.3-0.4 inches and preferably about 0.335 inches.
The formation of the above-described thread is difficult due to the relatively large amount of material which must be removed from the gap G between the threads. Moreover, the formation of the thread is complicated by the constant root and crest diameter of thread, especially on the tapering portion of the shank.
One method of forming the threads on a spike is by hot rolling (e.g., see British Patent Specification No. 757,709 filed May 12, 1953) in which a heated, blank or unthreaded spike is introduced between rollers having a complimentary thread to that for formation on the spike. The complimentary threads impress the red hot spike with their thread design. Hot rolling is costly in terms of energy consumption and not particularly accurate in terms of thread precision.
U.S. Pat. No. 4,278,374 discloses a milling apparatus for cutting a thread on a cylindrical railroad spike while tapering the shank. In that apparatus, a tool holder ring is rapidly and eccentrically rotated about a workpiece which is axially advanced relative to the tool holder. The tool holder ring supports a plurality of single point cutting elements disposed in a series of adjacent planes, the number of planes increasing with the degree of tapering in the shank. During each revolution of the tool holder ring, the cutting elements intermittently and successively tangentially contact the workpiece to chip away the material to form the thread. However, this milling technique does not form a proper thread on a railroad spike. In particular, the withdrawal of the single point cutters from the thread chips the spike to form a less precise thread with a poor run-out or thread ending. Moreover, the intermittent chipping operation is relatively slow and thus does not facilitate a smooth, economical operation. The milling apparatus is also difficult to set up and adjust.
In some screw threading operations, tangential chasers are preferable to milling and/or rolling since the chasers continuously contact the workpiece to "peel" material from the screw in a relatively quick operation. For example, it has been known to use tangential chasers in a screw threading operation in which a gap does not exist between the flanks of adjacent teeth (e.g., see U.S. Patent No. 2,067,593). However, tangential chasers have not been used to cut the thread on the Tirefond or North American railroad spike since a chaser has yet to be designed to accommodate both the large gap between the flanks of adjacent thread turns and the large diameter of the spike. Moreover, the thread requirements of a constant root and crest diameter on the tapering portion of the spike complicates the manufacture of the chasers since the cutting edge of each chaser must be manufactured individually.
It is a primary object of the present invention to provide a thread forming apparatus which obviates the above-identified disadvantages associated with the prior art.
It is a further object of the present invention to provide a thread forming apparatus having tangential chasers for cutting a thread with a relatively large gap between adjacent turns of the thread.
Another object of the present invention is to provide a thread forming apparatus having tangential chasers for cutting a thread having a constant root and constant crest diameter on a Tirefond or North American railroad spike.
It is a further object of the present invention to provide a thread forming apparatus having tangential chasers for cutting a thread having a constant crest diameter and root on a railroad spike while cutting the tapering portion of the railroad spike.
In accordance with the present invention, the apparatus for cutting threads in the railroad spike includes a tool holder having a central axis, and means for rotatably and axially advancing a blank spike relative to the tool holder along the central axis. The spike is advanced so that the tip of the shank is located forwardly of the head of the spike in an advancement direction of the spike.
A plurality of chasers are fixedly supported in the tool holder in a symmetrical arrangement about the central axis for tapering the shank while cutting a thread having a constant pitch, root and crest diameter. Each of the chasers has a serrated cutting edge profile that includes a series of cutting edges alternating with a series of thread forming notches. Each of the thread forming notches has a forward flank and a rearward flank intersecting at a crest of the notch. Forward and rearward flank angles are defined respectively between the forward flank angle and a normal to the central axis, and the rearward flank and the normal. Each of the notches has a constant root and forward and rearward flank angles different from an adjacent notch.
Each of the cutting edges is defined between a forward flank of one notch and a rearward flank of a preceding notch. The crests of adjacent notches are separated by one pitch, while the cutting edges have equal lengths each of which corresponds to the gap between adjacent turns of the thread. The series of cutting edges on each chaser penetrates successively deeper into the spike in the advancement direction of the spike.
Each of the plurality of chasers has a different cutting edge profile. A cutting edge in the series of cutting edges on one chaser is located rearwardly of and penetrates successively deeper than a corresponding cutting edge in the series of cutting edges on a preceding chaser. The forward and rearward thread cutting notches on one notch on one chaser are no greater than the forward and rearward flank angles on a corresponding notch on a preceding chaser. The chasers in accordance with the presently claimed invention permit a thread to be cut with a relatively large gap between adjacent turns of the thread, while accommodating the constant root and crest diameter of the thread across the tapering portion of the shank.